Not all volcanoes are created equal. Nor do they erupt equally. As hurricanes have the Saffir-Sampson scale, tornadoes have the Fujita scale, and earthquakes have the Richter scale, volcanoes have the Volcanic Explosivity Index.
The VEI was first proposed in 1982 by Christopher Newhall and Steve Self in a paper titled “The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism” published in JOURNAL OF GEOPHYSICAL RESEARCH.
Since that time, the VEI has been used to classify the size of an eruption, as well as to forecast the potential size of an eruption based on that volcano’s history. The index spans from 0 to 8 and measures the magnitude (erupted volume) and intensity (eruption column height) of an eruption.
Volcanologists determine a rating by observing an active volcano’s column height. When assessing historical eruptions, the rating is determined by the volume of deposits, the ash and lava left behind after the eruption.
The VEI numbers represent a range with some of those ranges overlapping. The most overlap occurs in the middle of the range. For example, a VEI 3 has a cloud column height of 3-15 kilometers and can last from 1 to 6 hours, whereas a VEI 4 has a cloud column height of 10-25 kilometers and can last from 1 to 12 hours. Below is a guide to the VEI and examples.
0 – non-explosive
These volcanoes are re-effusive rather than explosive, meaning that the lava flows rather than erupts from them. The Hawaiian volcanoes are examples of non-explosive ones. This rating on the VEI is considered gentle, with cloud column heights less than 0.1 kilometer in height and basically no material expelled into the atmosphere. There are approximately 20 – 30 of these eruptions each year.
1 – small
There is a VEI 1 eruption every day somewhere in the world. These volcanoes can be Hawaiian or Strombolian in nature with cloud columns less than 1 kilometer. Stromboli Colcano in Italy and Semeru Volcano in Indonesia are two examples of a small VEI volcano.
2 – moderate
VEI 2 eruptions occur weekly. Usu 2000 in Japan is an example of a VEI 2. This is the bottom of the range “explosive” with cloud column heights from one to five kilometers. These volcanoes can be strombolian or vulcanian in nature and will last one to three hours.
3 – moderate-large
This is the range where volcanoes start becoming quite dangerous and rare. There are approximately three VEI 3 eruptions each year. Mayon Volcano 2001 in the Phillipines and Pago Volcano 2002 in Papua New Guinea show the features of a VEI 3 volcano.
These exhibit cloud columns from 3 -15 kilometers, are explosive, vulcanian in nature (though some are plinian) and last generally 3-9 hours. They inject substantial material into the atmosphere.
4 – large
These destructive volcanoes only happen about once a year. Shishaldin and Augustine Volcanoes in Alaska both represent VEI 4 volcanoes. A VEI 4 is severely explosive to the point of cataclysmic, plinian eruptions.
The cloud column height is 10-25 kilometers, and the eruption can last 6 to 9 hours. Pyroclastic flows are common as are material expulsions into the stratospheric atmosphere.
5 – very large
Occurring once every 10 years or so, perfect examples of these catastrophic volcanoes are Mt. St. Helens’ 1980 eruption, Mt. Vesuvius’ 79 AD eruption and Mt. Chaitén’s 2008 eruption. These volcanoes have column heights exceeding 25 kilometers.
They are plinian eruptions with numerous pyroclastic flows. They inject significant amounts of material into the stratosphere. There have only been 106 of these in recorded history.
6 – paroxysmal
Krakatau in 1883 and Pinatubo in 1991 are examples of these terrifying volcanoes. The severity of the volcano is measured mostly by its duration, usually more than 12 hours, and amount of material ejected, over 1 billion meters cubed.
These occur approximately once every 100 years, but their effects can be felt globally and for years to come. Only 46 of these have erupted in recorded history.
7 – colossal
The good news is that these erupt only once every 1000 years and Tambora erupted in 1815. VEI 7 have long and far-reaching consequences. They expel over 10 billion cubic meters of material, making their effects felt worldwide as ash clouds block sunlight and lower world temperatures.
These are ultra-plinian explosions lasting more than 12 hours with devastating pyroclastic flows. Mankind has only experienced four of these volcanoes.
8 – mega-colossal
No mega-colossal volcano has ever been recorded. The only known one is Yellowstone which last erupted 600,000 years ago. A mega-colossal volcano expels over 100 billion cubic meters of material in ultra-plinian eruptions that are felt around the globe.
It is estimated that the explosion of a mega-colossal volcano will dip the Earth into a volcanic winter with decreasing temperatures, unexpected freezes, crop deaths and famine. This is a natural disaster of epic proportions.
Like all other natural disasters, volcanoes have a rating system in the VEI. Though not perfect, it gives volcanologists a good way to measure the possible eruptive nature and strength of any given volcano based on its history. With much overlapping in the ranges, volcano ratings can easily be changed as additional information is provided.
Throughout the year, scientists monitor volcanoes, their characteristics and take into consideration their location and weather patterns, all of which will help determine how localized, or global, a volcano’s effects will be.